1. Field of the Invention
The present invention relates to an image forming apparatus that forms an electrostatic latent image on an image carrier, forms a toner image by developing the electrostatic latent image with respective color toners, and forms a color image on a transfer material by transferring and fixing the toner image to and on the transfer material through an intermediate transfer member.
2. Description of the Related Art
Heretofore, the above type of image forming apparatuses have become widespread as image forming apparatuses for forming, for example, a high quality color image.
As one of the image forming apparatuses, there is known a so-called tandem type image forming apparatus in which four sets of image carriers are disposed in correspondence to four colors of cyan (C), magenta (M), yellow (Y), and black (K), each of the image carriers shares formation of a toner image of one of the colors, and the toner images of the four colors are superimposed on an intermediate transfer unit and transferred onto a sheet at a time (refer to, for example, Japanese Patent Application Laid-open No. 2003-156907). In the tandem type image forming apparatus, since the respective image carriers share the formation of the toner images of the respective colors, the toner images of the respective colors can be simultaneously formed, and thus the image forming apparatus is suitable to form an image at a high speed. In the tandem system, however, a problem arises in that the size of the image forming apparatus is increased because various components as many as the four image carriers must be disposed around the image carriers and further a large intermediate transfer unit, which moves across all of the four image carriers, is required. Further, a problem also arises in that since any unit except a fixing unit is disposed on the side of the large-sized intermediate transfer unit opposite to the side thereof where the image carriers are disposed, a large dead space occurs on the side where only the fixing unit is disposed.
In contrast, there is also known an image forming apparatus of a type that employs only one set of an image carrier to reduce the size and the cost of the apparatus and causes the one image carrier to sequentially form toner images having the respective colors of C, M, Y, K (refer to, for example, Japanese Patent Application Laid-open No. 10-339983). This type of the image forming apparatus forms an image at a low speed because the toner images of the respective color toners must be sequentially formed by the one image carrier. However, since an image is formed using only a black toner more frequently on the average, this type of the image forming apparatus has been also accepted from a view point of the reduction in size and cost of the apparatus.
FIG. 21 is a view showing an arrangement of an example of the image forming apparatus shown in Japanese Patent Application Laid-open No. 10-339983.
A photosensitive drum 101 is charged by a charging unit 102 while rotating in the direction of an arrow A about the center of rotation 101a thereof, and an electrostatic latent image is formed on the photosensitive drum 101 by the exposure light 103a from an exposure unit (not shown) and developed by a rotary developing device 104.
The rotary developing device 104 has plural developing units 104C, 104M, 104Y, and 104K disposed around the center of rotation thereof to execute development using respective color toners, and the electrostatic latent image formed on the photosensitive drum 101 is developed by a developing unit (the developing unit 104Y in the state shown in FIG. 21) which is faced to the photosensitive drum 101 by the rotation of the rotary developing device 104.
A toner image is formed on the photosensitive drum 101 by being developed by the rotary developing device 104 and transferred onto an intermediate transfer belt 106, which circulatingly moves in the direction of an arrow B, by the action of a transfer unit 105 at a primary transfer position 118. After the toner image has been transferred onto the intermediate transfer belt 106, the surface of the photosensitive drum 101 is cleaned by a cleaner 119, a toner image forming operation is executed again by starting the operation from the charge of the photosensitive drum 101 executed by the charging unit 102.
When a color image is formed, the above image forming cycle is repeated four times, thereby a toner image composed of four colors of cyan (C), magenta (M), yellow (Y), and black (K) is formed on the intermediate transfer belt 106.
In contrast, a sheet P is fed from a sheet accommodation unit 107 by a pick-up roll 108, conveyed by conveying rolls 109 along a conveying path 110, sent to a secondary transfer position 112 by registration rolls 111 in exact timing with the toner image formed on the intermediate transfer belt 106 and composed of the four CMYK colors. At the secondary transfer position 112, the four-color toner image on the intermediate transfer belt 106 is secondarily transferred onto the sheet by the action of the transfer unit 113.
The sheet, which has undergone the transfer of the four-color toner image, is conveyed by a conveying belt 114 while carrying the four-color toner image, clamped between a pair of fixing rolls 115a disposed on a fixing unit 115 and heated and pressurized thereby, and the toner image is fixed on the sheet.
The sheet, on which the toner image has been fixed, is further conveyed by conveying rolls 116 and discharged externally of a cabinet of the image forming apparatus by sheet discharge rolls 117.
In this type of the image forming apparatus, it is also one of large themes to more reduce the size of the apparatus. Thus, the arrangement of the image forming apparatus shown in FIG. 21 will be examined from a view point of reduction in size.
In the arrangement shown in FIG. 21, the secondary transfer position 112 between the intermediate transfer belt 106 and the transfer unit 113 is disposed upstream of a vertical line passing through the center of rotation 101a of the photosensitive drum 101 in a sheet conveying direction. This is one of effective points from the view point of the reduction in size. In the arrangement of FIG. 21, however, the primary transfer position 118 is disposed downstream of the secondary transfer position 112 of the intermediate transfer belt 106 (a portion where the intermediate transfer belt moves in a direction where it separates from the secondary transfer position 112). In this case, the length of the sheet conveying path is increased in a lateral direction, which is very disadvantageous in the reduction of size.
Japanese Patent Application Laid-open No. 2000-298382 proposes an image forming apparatus in which a primary transfer position is disposed upstream of a second transfer position of an intermediate transfer belt (a portion where the intermediate transfer belt moves toward the secondary transfer position).
FIG. 22 is a view showing an arrangement of the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2000-298382.
In this image forming apparatus, the components that execute the same operations as those of the components shown in FIG. 21 are denoted by the same reference numerals even if they are formed in different shapes and disposed differently for easy understanding. This is also applied likewise to the components explained later with reference to respective figures.
A photosensitive drum 101 is charged by a charging unit 102 while rotating in the direction of an arrow A about the center of rotation 101a thereof, and an electrostatic latent image is formed on the photosensitive drum 101 by the exposure light 103a from an exposure unit 103 and developed by a rotary developing device 104.
A toner image is formed on the photosensitive drum 101 by being developed by the rotary developing device 104 and transferred onto an intermediate transfer belt 106, which circulatingly moves in the direction of an arrow B, by the action of a transfer unit 105 at a primary transfer position 118. After the toner image has been transferred onto the intermediate transfer belt 106, the surface of the photosensitive drum 101 is cleaned by a cleaner 119, and a toner image forming operation is executed again by starting the operation from the charge of the photosensitive drum 101 executed by the charging unit 102. Waste toners, which have been removed from the photosensitive drum 101 by the cleaning executed by the cleaner 119, are accommodated in a waste toner tank 120.
When a color image is formed, the above image forming cycle is repeated four times, thereby a toner image composed of four colors of cyan (C), magenta (M), yellow (Y), and black (K) is formed on the intermediate transfer belt 106.
In contrast, a sheet P is fed from a sheet accommodation unit 107 by a pick-up roll 108, conveyed by conveying rolls 109 along a conveying path 110, sent to a secondary transfer position 112 by registration rolls 111 in exact timing with the toner image formed on the intermediate transfer belt 106 and composed of the four CMYK colors. At the secondary transfer position 112, the four-color toner image on the intermediate transfer belt 106 is secondarily transferred onto the sheet by the action of the transfer unit 113.
The sheet, which has undergone the transfer of the four-color toner image, is clamped between a pair of fixing rolls 115a disposed on a fixing unit 115 while carrying the four-color toner image and heated and pressurized thereby, and the toner image is fixed on the sheet.
The sheet, on which the color image has been formed by fixing the toner image thereon, is further conveyed and discharged onto a sheet discharge tray 121 disposed externally of a cabinet of the image forming apparatus by sheet discharge rolls 117.
In the image forming apparatus shown in FIG. 22, the primary transfer position 118 is disposed upstream of the secondary transfer position 112 of the intermediate transfer belt 106 (a portion where the intermediate transfer belt moves toward the secondary transfer position), which will greatly contribute to reducing the size of the image forming apparatus. Further, the secondary transfer position 112 is disposed upstream in a sheet conveying direction of a horizontal line E that passes through the center of rotation 101a of the photosensitive drum 101, which also acts advantageously to the reduction in size of the image forming apparatus.
However, the pair of fixing rolls 115a, which constitute the fixing unit 115, for example, are disposed at positions that protrude downstream (upper side) in the sheet conveying direction of any of a horizontal line F, which is in contact with a portion of the intermediate transfer belt 106 on the most downstream side thereof in the sheet conveying direction, and a straight line, which connects the center of rotation 101a of the photosensitive drum 101 to the center of rotation 104a of the rotary developing device 104. Further, the charging unit 102 is disposed at a position that protrudes upstream (lower side) in the sheet conveying direction of a horizontal line H, which is in contact with a portion of the intermediate transfer belt 106 on the most upstream side thereof. Accordingly, the image forming apparatus has many improving points to reduce the size thereof.
Although the image forming apparatus is shown inaccurately in FIG. 22 because it is illustrated schematically, it is not preferable to curve the toner image on the intermediate transfer belt 106 at a small radius of curvature at the secondary transfer position 112 because disturbance of the image, and the like may be caused thereby. Further, it is also not preferable for the sheet sent from the secondary transfer position 112 to approach the intermediate transfer belt 106 (the toner image on the intermediate transfer belt 106) at a large angle and come into contact it, because disturbance of the image and the like may be also caused thereby. To cope with the above problem, in the arrangement shown in FIG. 22, a support roll 122, which supports the intermediate transfer belt 106 from the inside thereof, must be composed of a large diameter roll, and further a sheet conveying path must be set to permit a sheet to be supplied to the secondary transfer position 112 from the direction of an arrow C in order to reduce a contact angle between the intermediate transfer belt and the sheet.
In this case, when the sheet is curved strongly along a small radius, it may undergo a jam and the like. Thus, the length of the sheet conveying path is increased to supply the sheet to the secondary transfer position 112 from the direction of the arrow C in a state that it curves gradually in a large amount.
Therefore, actually, the length of the sheet conveying path is more increased upwardly of the secondary transfer position as well as the support roll 122 has a larger diameter as compared with the arrangement schematically shown in FIG. 22. As a result, the size of the apparatus is larger than that schematically shown in FIG. 22, which is contrary to the requirement for the reduction in size of the apparatus.